1. Field of the Invention
The present invention relates to new binder compositions based on copolymers having carboxylic anhydride and epoxide groups and organic compounds having at least two hydroxyl groups per molecule and to their use in coating compositions and sealing compositions.
2. Description of the Prior Art
The use of two-component compositions based on organic compounds having at least two intramolecular carboxylic anhydride groups per molecule and polyhydroxyl compounds as binders for paints and coating compositions is known.
FR-A-2,393,092 describes coating compositions based on hydroxy-functional polymers and an ester anhydride having at least two anhydride groups per molecule, the ester anhydride being a derivative of trimellitic anhydride.
EP-A-48,128 describes coating compositions based on a hydroxyl component having at least two hydroxyl groups per too molecule, an anhydride component having at least two anhydride groups per molecule and, optionally, an amine component acting as catalyst.
The disadvantage of these systems is that their solvent resistance is inadequate for high-quality paints.
Binder compositions based on binder components having carboxyl groups and epoxide groups, which may be cured both at room temperature and also under stoving conditions, are already known. Thus, DE-A-2,635,177 and DE-A-2,728,459 describe low-solvent stoving enamels containing a polyacrylate having carboxyl groups, an epoxy resin having at least two epoxide groups per molecule and a solvent mixture. The disadvantage of these systems is the preliminary reaction between the free carboxyl groups and free epoxide groups which can lead to an inadequate pot life of the paints.
International patent application, No. WO 84/00768, describes heat-curing coating compositions based on a polyacrylate containing epoxide and hydroxyl groups, a special dicarboxylic anhydride and an amine resin crosslinker. However, the anhydride component, which acts as a curing agent, contains only one carboxylic anhydride group per molecule. In addition, the systems have to be cured in the presence of hardening catalysts at high temperatures (approx. 130.degree. C). International patent applications, Nos. WO 84/00770 and WO 84/00771, describe comparable systems with the difference that the first component contains only hydroxyl groups and a separate component containing epoxide groups is additionally present.
The use of three-component systems based on organic compounds having at least two intramolecular carboxylic anhydride groups per molecule, polyhydroxyl compounds and polyepoxide compounds as binders for paints and coating compositions is also known (EP-A-134,691, EP-A-316,874 and EP-A-358,306). These three-component systems are superior to the above-cited two-component systems in regard to the solvent resistance of the paint films, but have the disadvantage that all three components must be completely compatible with one another to guarantee a good optical appearance of the paint film. This limits the range of variation of the individual components to a considerable extent.
EP-A-319,864 describes binder compositions based on a copolymer component containing incorporated carboxylic anhydride and incorporated epoxide groups and a polyamine component containing blocked amino groups. This reference does not teach or suggest the presence of a component containing hydroxyl groups.
Accordingly, an object of the present invention is to provide new binder compositions which
1) cure through addition reactions which take place between carboxylic anhydride, hydroxyl and epoxide groups, PA0 2) have an adequate pot life for paint applications, 3) depending on the particular application can be cured at room temperature or at elevated temperature to form clear, colorless, yellowing-resistant and solvent-resistant coatings and PA0 4) do not have any of the disadvantages of the previously described systems. PA0 A) 10 to 99 parts by weight of a copolymer component containing at least one copolymer prepared from olefinically unsaturated compositions having a weight average molecular weight of 1,500 to 75,000 and having in chemically bound form both PA0 B) 1 to 90 parts by weight of a hydroxyl component containing at least one organic polyol having at least two hydroxyl groups per molecule, PA0 a) olefinically unsaturated monomers containing anhydride groups,, PA0 b) olefinically unsaturated monomers containing epoxide groups and PA0 c) non-functional, olefinically unsaturated monomers free from anhydride and epoxide groups.
This object may be achieved in accordance with the binder compositions of the present invention as described in detail hereinafter.
The binder compositions according to the invention contain a component A) having both cyclic carboxylic anhydride groups and also free epoxide groups in chemically bound form, and a component B) having at least two free hydroxyl groups per molecule. These binder compositions have never been described before. The particular advantage of these systems is that the reactive groups are present in the form of carboxylic anhydride, epoxide and hydroxyl groups in only two components A) and B) and that component A) does not initially contain the free carboxyl groups which are necessary for crosslinking the epoxide groups of component A). Instead these carboxyl groups are only formed from the reaction of the carboxylic anhydride groups with the hydroxyl groups of component B) by the action of catalysts or heat.
Accordingly, these binder compositions show considerably better storage stability at room temperature than comparable prior art systems which contain free carboxyl groups. Another advantage of the binder compositions according to the invention is that the crosslinking reaction is based on a so-called "double-cure" mechanism, i.e., the hydroxyl groups of component B) initially react with the anhydride groups of component A) with opening of the anhydride ring and formation of carboxyl groups. This reaction is itself a crosslinking reaction. In addition, however, the newly formed carboxyl groups can react with the epoxide groups of component A) which results in additional crosslinking.
By virtue of the high crosslink densities which may be obtained, coatings characterized by very high resistance to chemicals and solvents are obtained from the binder compositions according to the invention.